1. Field of the Invention
The present invention relates to an image signal processing method for processing an image having a continuous tone and to an apparatus for practicing the method.
2. Description of the Prior Art
Common documents such as newspapers, magazines, business reports and the like composed of characters, linetone images, photographs etc.
Characters and linetone images are composed of lines with little tonal rendition and require greater sharpness than images with tonal rendition since importance is given to legibility transmitting the meaning of the images.
On the other hand, photographs and pictures are composed of image components rich in tonal rendition, and emphasis of such tonal rendition is important since such images transmit emotional information.
Further an image field is often composed of a group of images requiring different image qualities.
In order to form such an image, including portions having a continuous tone, with binary representation, there are already known dot-pattern methods and dither methods. In the dot-pattern methods the gradation of the image is represented by dividing the image into small sections and changing the area of a micro-element, such as a dot or a line, in each section.
In the dither methods, the gradation is represented by changing, independently from the image, the threshold value for digitalizing the signals of an image divided into small sections. For example in the so-called systematic dither method, an image composed of a matrix arrangement of picture elements or pixels is divided into sub-matrices each composed of n.times.n pixels, and a dither pattern corresponding to the dimension of each sub-matrix is prepared and utilized as the threshold value for digitizing the signals of the pixels.
For an image displaying device such as a liquid crystal device or ink jet printer in which pixels of a determined shape arranged in a matrix are controlled by electric signals to be respectively on or off, processing with the dither method, particularly the systematic dither method, is most suitable.
The conventional systematic dither method, however, is unsatisfactory since the edge characteristic, particularly the sharpness in representing tonal rendition, and in producing moire stripes, in the use of n.times.n dither pattern, is sacrificed due to the interference between the periodicity of the image and the periodicity of digitizing threshold values repeated by n pixels.